In dentistry, a dental prosthesis is produced with the aid of models. Such a working model must reproduce the teeth and jaw true to the original. A three-dimensional negative of the jaw situation is first produced with the aid of an impression material. For this, the impression material, in its plastic state, is introduced into the mouth of the patient with an impression tray, and solidifies there to form an elastic composition, which is the negative mould after removal. The working model is obtained by filling this impression with a modelling material. Impression materials which are used are, for example, compositions based on alginates, polysulphides, silicones and vulcanizable polyether materials.
These compositions have various properties and have advantages and disadvantages for each type of use on the basis of their processability, their physical properties, their quality of impression and taking reproduction and their cost.
Such impression materials which can be vulcanized at room temperature are prepared by mixing a composition comprising one or more components, usually and more favourably two, at least one base component and one hardener or initiator or catalyst component. The base component and hardener are basic components of the composition. The rubber-elastic solidification which occurs after mixing is called vulcanization, polymerization, setting or curing.
Impression materials of synthetic polymers based on polysiloxanes (silicones) are widespread. They are crosslinked to an elastomer by a chemical reaction. In the case of silicone materials, a distinction is made between condensation-crosslinking and addition-crosslinking silicone compositions. The condensation-crosslinking silicones as a rule crosslink by titanium- or tin-catalysed reaction of the polysiloxane having hydroxyl end groups with alkoxysilicone compounds, an alcohol being split off. In the case of the addition-crosslinking silicones, crosslinking is effected as a rule by platinum-catalysed reaction of unsaturated hydrocarbon end groups of the polysiloxane with Si--H groups of a hydridopolysiloxane (hydrosilylation). Such silicone impression materials are known, for example, from EP-A-0 162 211, DE-A-40 31 759 and EP-A-0 166 107. The silicone compositions are as a rule supplied as a two-component system, and after the components have been mixed, the composition formed is introduced into the mouth of the patient and cures there within a period of usually 3-5 minutes.
One disadvantage of the known vulcanizable compositions is that they have a relatively low dimensional stability in the uncured state. Although this can be improved by compounding with suitable, chemically compatible fillers and additives (for example addition of pyrogenic silicic acids or a high degree of filling of the paste with coarse inorganic fillers), this is also usually associated with thixotropic properties or a loss of detail reproduction of the paste while taking the impression. This means that while applying pressure in taking the impression, either the materials tend to yield to the pressure and flow away or fine details cannot be reproduced. Because of this property, the penetration capacity of the silicone impression materials into gaps, which is important for the reproduction of the details in the mouth, is not particularly good.